The field of cutting tools employing indexable lay down inserts encompasses a wide range of tools including face mills, step mills, end mills, boring tools and turning tools to name a few. These tools incorporate an equally varied array of cutting edge designs to accommodate both the operational parameters of the tools and the production specifications of the workpiece. Where it is desirous to remove large volumes of material (rough cutting) by face milling, the cutting edge has been designed to withstand higher loading. For example, a face milling tool employed to rough cut carbon steel, driven by a 30-50 horsepower machine may operate at a feed rate of 0.015 inches per tooth at a 0.125-0.250 inch depth of cut utilizing a surface feed rate of 200-300 feet per minute. Smooth finish milling, however, places different conditions on cutting edge design. A face mill equipped with finishing inserts and operated in the same machine and in the same material above runs at a lower feed rate of 0.005 inch - 0.008 inches per tooth and at 0.020-0.030 inch depth of cut at 300-400 feet per minute. Although the cutting edge in finish milling is not required to withstand the same load requirements as in rough cutting, the edge must provide a considerably smoother surface finish. Values in the range about 125 RMS for fine surface milling in comparison with about 250 RMS for rough cutting are not uncommon.
Several attempts have been made to improve cutting edge performance in both rough and finish cutting tools by changing the orientation of the cutting edge with respect to the tool seat. Though varied, the approaches have included the selection of a positive or a negative rake design.
Negative rake cutting tools provide an insert seat which is inclined at a negative rake angle relative to the cutting plane and a straight sidewall form. The inclination of the insert seat assures clearance under the cutting edge.
Positive rake cutting tools provide inserts fixed to inclined seats in such a manner that inserts are required to have an inclined sidewall form to provide clearance under the cutting edge. The back wall of the insert must be inclined rearwardly to complement the sidewall form of the insert. This provides a ramp surface rather than a pocket at the back wall of the insert and requires additional means for fastening the insert. Most positive rake inserts provide only half the number of available cutting edges because they cannot be indexed end over end. Use of a negative rake insert is therefore desirable. Examples of art employing positive rake inserts include U.S. Pat. Nos. 3,938,231 and 3,868,752.
It is also known in the art to modify the orientation of the cutting edge with respect to the cutting tool and workpiece by employing positive or negative radial and axial rake angles.
Generally, the term "rake" is the angular relationship measured between a reference plane and a reference face of the insert. The reference plane passes through the cutter body central line axis and the inserts cutting corner. The reference face sometimes referred to as rake face or first surface herein is the face that sees the work piece and is dependent upon the direction of cutter rotation.
The inserts radial rake angle is the angle formed by the referenced plane and the rake face as measured in the plane perpendicular to the cutter body axis.
Radial rake is defined as positive where the rake face forms an acute angle with respect to the reference plane such that the rake face slopes away from the direction of cutter rotation when applied to a workpiece. Radial rake is defined as negative where the rake face forms an obtuse angle with the reference planes and slopes toward the direction of cutter rotation. Generally, a negative rake is preferred in applications where the cutting edge is be subject to high loading.
The insert's axial rake connotes the angle formed between the reference plane and the rake face measured in a plane perpendicular to the radius of the cutting body, at the working cutting corner. The use of negative radial rakes in combination with negative or positive axial rakes is known in the art. One example is found in U.S. Pat. Nos. 3,289,271.
It is also known in the art to further define the orientation of a cutting edge in terms of its true rake angle and angle of inclination or true shear. The true rake angle is defined by drawing an imaginary line normal to the cutting edge and intersecting the axis of the cutter body. The angle between the rake face and this imaginary line is the true rake angle.
The angle of inclination is defined by drawing an imaginary line through the center point of the cutter body and tangent to the radially outermost point of the cutting edge. The angle between this line and the cutting edge is the angle of inclination.
If the plane of the rake face passes through the cutter axis, the true rake is said to be zero. If the top working corner of the cutting edge is ahead of the lowermost point on the cutting edge, the true rake is said to be positive. If the radially outermost point of the cutting edge passes through the cut first, then the inclination angle is said to be negative.
GTE Valenite U.S. Pat. No. 4,352,609 discloses a face milling cutter and a cutting edge with a positive true rake angle in the range of 0.degree. to 3.degree. and a radial rake on the order of 0.degree. to 2.degree. positive, with an axial rake on the order of 4.degree. to 6.degree. negative. A cutting edge utilizing a positive radial rake with a negative axial rake, however, tends to not only fracture under heavy loading but also directs spent chips toward the workpiece. This results in recutting and scouring, requiring greater horse power per cubic inch of stock removed.
Attempts at modifying the performance characteristics of cutting tools has also included incorporating a lead angle. The term lead angle is known in the art and is defined as an angle formed between the radially outward facing insert edge that includes the working cutting corner and an imaginary line oriented parallel to the cutting body axis that passes through the cutting corner.
A cutting tool, which has high efficiency in both soft and difficult to machine materials and facilitates the formation of tight chip for rapid removal and also generates a smooth surface at high feed rates would be a desirable advance in the art of cutting tools. By optimizing the radial and axial rakes of the insert, the extent of the lead angle, true rake and angle of inclination, the problem of stocking multiple cutters and inserts for rough cutting and finishing is alleviated.
One object, therefore, in the present invention is to provide a cutting tool employing lay down indexable inserts which includes an enhance cutting edge and which generates short, tightly curled chips.
A further object of the present invention is to provide a cutting tool employing a lay down indexable insert which can withstand high feed rates in difficult to machine materials without failure and simultaneously deliver a smooth final finish on the order of 125 RMS or better.
A further object of the present invention is to provide a cutting tool employing a lay down indexable insert which is easy to manufacture and where the dies for the insert are made according to standard machining practices.